Publicaciones: Sistemas Hibridos Nanoestructurados

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20

Nov 2017

2D magnetic domain wall ratchet: The limit of submicrometric holes

Posted by / in Publicaciones: Sistemas Hibridos Nanoestructurados, Publications 2018, Publications Nanostructured Hybrid Systems 2018, Últimas Publicaciones CINN / No comments yet

The study of ratchet and crossed-ratchet effects in magnetic domain wall motion through 2D arrays of asymmetric holes is extended in this article to the submicrometric limit in hole size (small size regime). Therefore, the gap has been closed between the 2D ratchets in the range of tens-of-micrometers (large size regime) and the small size regime 1D ratchets based on nanowires. The combination of Kerr microscopy, X-ray PhotoEmission Electron Microscopy and micromagnetic simulations has allowed a full magnetic characterisation of both the domain wall (DW) propagation process over the whole array and the local DW morphology and pinning at the holes. It is found that the 2D small size limit is driven by the interplay between DW elasticity and half vortex propagation along hole edges: as hole size becomes comparable to DW width, flat DW propagation modes are favoured over kinked DW propagation due to an enhancement of DW stiffness, and pinned DW segments adopt asymmetric configurations related with Néel DW chirality. Nevertheless, both ratchet and crossed-ratchet effects have been experimentally found, and we propose a new ratchet/inverted-ratchet effect in the submicrometric range driven by magnetic fields and electrical currents respectively.

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24

Jul 2017

Deterministic propagation of vortex-antivortex pairs in magnetic trilayers

Posted by / in Publicaciones 2017, Publicaciones Sistemas Híbridos Nanoestructurados 2017, Publicaciones: Sistemas Hibridos Nanoestructurados, Últimas Publicaciones CINN / No comments yet

Thin perpendicular magnetic anisotropy films between two soft ferromagnetic layers have the nuclei for magnetization inversion at the bifurcations of their characteristic stripe domain pattern. The inverted nuclei induce vortex-antivortex pairs in the soft magnetic layers that exhibit a correlated motion extending several μm along the magnetic stripes during magnetization reversal.The sense of motion is completely determined by the topology of the magnetic bifurcations causing vortex-antivortex pairs to propagate in opposite senses depending on their polarities. This is a robust effect that might have practical applications. These findings are based on X-ray microscopy and micromagnetic calculations.

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22

Jun 2017

Perpendicular magnetic anisotropy in amorphous NdxCo1−x thin films studied by x-ray magnetic circular dichroism

Posted by / in Publicaciones 2017, Publicaciones Sistemas Híbridos Nanoestructurados 2017, Publicaciones: Sistemas Hibridos Nanoestructurados, Últimas Publicaciones CINN / No comments yet

The origin of perpendicular magnetic anisotropy (PMA) in amorphous NdxCo1−x thin films is investigated using x-ray magnetic circular dichroism (XMCD) spectroscopy at the Co L2,3 and Nd M4,5 edges. The magnetic orbital and spin moments of the 3d cobalt and 4f neodymium electrons were measured as a function of the magnetic field orientation, neodymium concentration, and temperature. In all the studied samples, the magnetic anisotropy of the neodymium subnetwork is always oriented perpendicular to the plane, whereas the anisotropy of the orbital moment of cobalt is in the basal plane. The ratio Lz/Sz of the neodymium 4f orbitals changes with the sample orientation angle, being higher and closer to the atomic expected value at normal orientation and smaller at grazing angles. This result is well explained by assuming that the 4f orbital is distorted by the effect of an anisotropic crystal field when it is magnetized along its hard axis, clearly indicating that the 4f states are not rotationally invariant. The magnetic anisotropy energy associated to the neodymium subnetwork should be proportional to this distortion, which we demonstrate is accessible by applying the XMCD sum rules for the spin and intensity at the Nd M4,5 edges. The analysis unveils a significant portion of neodymium atoms magnetically uncoupled to cobalt, i.e., paramagnetic, confirming the inhomogeneity of the films and the presence of a highly disordered neodymium rich phase already detected by extended x-ray-absorption fine structure (EXAFS) spectroscopy. The presence of these inhomogeneities is inherent to the evaporation preparation method when the chosen concentration in the alloy is far from its eutectic concentrations. An interesting consequence of the particular way in which cobalt and neodymium segregates in this system is the enhancement of the cobalt spin moment which reaches 1.95 μB in the sample with the largest segregation.

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